From: ccie2be (ccie2be@nyc.rr.com)
Date: Wed May 26 2004 - 19:47:10 GMT-3
Hey Sam,
Although the way this stuff is explained in much of the documentation, it's
really not that conceptually difficult. Here are some of my notes from the
IE bootcamp I took (BTW, if you're in a hurry to pass the lab, I highly
recommend you sign up for this class. The 2 Brian's are fantastic
instructors.)
-STP: Cost is determined the same way for STP as it is for IGP's: Cost =
cost of local int plus the cost of all int's pointing upstream towards
destination.
-If there are multiple trunks connecting the 2 Cat's & each trunk is allowed
to carry traffic for all vlan's (the default), STP will block at least one
interface. Assume Cat1 is the root bridge for vlan X. Once Cat1 is set (or
elected) Root Bridge, Cat2 must decide which of it's ports is its root port
to Cat1.
1. Cat2 will compare the port cost of each trunk interface (by default,
they're all equal)
2. Cat2 will compare BID's from each trunk - of course, they're equal
because both BID's are from Cat1.
3. Cat2 will then compare the port priority for each link from Cat1.
4. Last, Cat2 will use its own port's ID as the last tiebreaker
-As a result of above process, if you have to config a particular link to be
in forwarding mode & you can't config Cat2 to do this, then change Port
Priority on Cat1.
-If the change can only be made on Cat2, lower port cost on Cat2. Notice
that if you're restricted to modifying either port cost or port priority,
that restriction determines on which Cat you have to make the change.
Also, notice the order of the steps above after the root bridge is elected.
The first step Cat 2 using to determine which interface will be the root
port will be to compare the cost of each trunk interface to the root bridge.
Now, since cost is determined according to that rule shown above, the cost
that Cat 2 is comparing is the cost set on it's own interfaces. Therefore,
if the port cost on one of the interfaces is changed while the cost of the
other port is left at it's default, Cat 2 will make the port with the lower
cost the root port and after that no other comparison's are done (or
needed).
From the point of view of passing the lab, the thing to realize is that
there are different ways to skin a Cat. :-)
What you need to pay attention to is what restrictions are placed on you.
If you're only allowed to make the change on a specified Cat, then you need
to know which parameter to use (port priority or port cost ). If you're only
allowed to change one of the parameters, then you need to know which Cat to
make the change on.
Lastly, don't forget that all these parameters are, by default, on a per
vlan basis. This can be useful if you want to load balance your traffic.
For example, let say you want vlan's 1 - 10 to use port X as the root port
and vlan's 11 - 20 to use port Y as the root port. Assuming Cat 1 is the
root bridge for all vlan's, you'll set the cost (or priority) to accomplish
that task. Now, here's the good news: If one of the links goes down, STP
will automatically reroute traffic for those vlans which were using the link
that went down. It will do this by UN-blocking the port for those vlans
which were previously using the now down port.
HTH
----- Original Message -----
From: <samccie2004@yahoo.co.uk>
To: "studygroup" <ccielab@groupstudy.com>
Sent: Wednesday, May 26, 2004 1:47 PM
Subject: Port Priority" Vs "Path Cost
> Hi Group
>
> Though I read TAC, I am still unclear about what is different between the
2
> options.
> When do u prefer "Port Priority" Vs "Path Cost" and vice versa. They seem
to
> both achieve the same results.
>
> TIA
>
> Sam
>
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